A lot of underwater LED lights are manufactured all over the world and in our country including. How to choose and bye the really worthwhile thing in this great variety?
We will try to help you, but it's for you to decide:
1. What is the light-emitting diode and its efficiency?

Light-emitting diode – is a semiconductor solid-state light source.

LEDs are classified as illuminating and indicating depending on efficiency. In our lights we use both of them.

Light output or light source efficiency is determined as radiated light (lumen) for 1 Watt of consumed power. The measuring unit is lm/W.

Some LED manufacturers have already approached in terms of efficiency to the most efficient light sources – sodium lamps. You can see hundred thousands of yellow sodium lamps along roads. They have light output up to 150 lm/W, but they aren't reliable, they need high voltage and aren't suitable for our needs.

Here are the most well known global manufacturers of the powerful LEDs: Nichia (Japan), Lumileds (USA), Seoul Semiconductor (Korea), Edisson Opto (Taiwan), ProLight Opto (Taiwan), Cree (USA).

The most efficient and high-quality LEDs for today are serial produced by the Cree Company. It also supplies 80% of the global market with its chip-crystals to other LED manufacturers. Up-to-date, serial produced LEDs by this company have minimum light output of 140 lm/W.

2. How to know the true luminous flux of the underwater light?

Most manufacturers of underwater LED lights, even well-known, purposely overstate the luminous flux value in chase of money. Many of them overstate in many times! They use the fact that most of the users have no possibility to check the luminous flux, since the lumometer is the expensive and uncommon device.

How to reveal the deceit? An approximate value can be just calculated:

Step 1 Determine the power capacity of the battery. 
To do so find its type in the light documents. Average canister (6-8 cm in diameter and 20-25 cm in length) comprises up to 40 W*h for NiMH batteries or 70-100 W*h for Li-Ion batteries.
You can find it out more exactly for your light if the documents are available. To calculate the power capacity you should multiple element voltage (1.2 V for NiCd and NiMH; 3.7 V for Li-Ion) to its capacity and to number of elements. For example, in your light there are 8 of AA NiMH elements with 2500 mAh capacity (=2.5 A*h). In this case the battery power capacity will be 1.2 V х 2.5 Ah х 8 = 24 W*h.

Step 2 Determine the power demand of the light.
To do so find in documents the specified time of operation at the full power. For example the specified time is 10 hours. Divide the battery capacity by the operation hours. Divide 24 W*h by 10 h, and get that the light consumes 2.4 W.

Step 3 Determine the average LED power.
Usually about 15% are spent for the power conversion before LED. So, multiple the light power consumption to 0.85. If the light consumes 2.4 W, so its LEDs consume about 2.4 х 0.85 = 2.04 W.
You can calculate the capacity of each LED by dividing their total power by their quantity. For example, if there are 3 LEDs, the capacity of one of each is 2.04 / 3 = 0.68 W. Please note at this point, that if the calculated LED power value very discriminate from known value (for example, you know that your light is based on three-watt LEDs and calculate value is 0.68W), then the manufacturer overstates full-power continuous operation time very much. Though if the light keeps the specified time, then the LED power decreases significantly within this period. So, you have calculated theaverage value.

Step 4 Determine the luminous flux.
Assume that the light has the best LEDs with the light output of 130 lm/W. If there is not more than 1.5 W for 1 LED, you can just multiple the light output by the capacity. I.e. 130 х 2.04 = 265 lumens (compare this result with the specified by the manufacturer and draw a conclusion). The other situation is possible – when the battery is fully charged the luminous flux is close to the specified, but at the end of the operation time it’s ten times less, and such light is useful only to backlight devices.

If there is about 2-3 W for 1 LED, the light output is less than 100 lm/W. The more LED capacity, the less its light output.

3. What is the light continuous operation time?

Internal power supply of the light should provide LEDs constant current during the whole specified operation time. Only in this case the light will always have constant brightness.
The trick is that human eye adapts and doesn’t notice gradual and slow brightness decrease. Many manufacturers use this effect to overstate the operation time at maximum power, sometimes in several times (!!). In fact it’s enough to take two same lights: one with fully charged battery, and second – used for the specified time at the “maximum power”, and compare them lighting. The difference is often impressive.

As a rule, luminous flux of such light starts decreasing as early as half of the charge, and in some cases it starts actually at once. The light fades with decreasing of the battery voltage. This allows for significant increasing of the operation time. The luminous flux of such lights can be described only tentatively. The above mentioned method helps to calculate some average luminous flux value for such case.

In our underwater lights such mode called emergency mode and turned on automatically at deep discharge of the Rechargeable battery (more than 80%). The mode is displayed by the indicator at once.

4. What kind of optics is used for LEDs and LED lights?

Among important requirements to the underwater lights for diving are central spot radiation density and relatively narrow beam. High central spot radiation density helps to minimize effect of light reflection in muddy water and illuminate objects at greater distance. But the beam should surely comprise a diffused part (side light) to form a “volume”.

Narrow radiation angle is more difficult to achieve than wide one. The light efficiency depends on quality and accuracy of the used optics. The result is also depends on relation between diameter of the lens (reflector) and size of the light source, as far as the source itself has significant dimensions and is not a infinitesimal point (as it is in the ideal model of the optical system). So the less diameter of the lens the less her efficiency.

For the CREE LEDs they produce collimating lens of middle size (20-26 mm) not less than 6 degrees. Some of them can be modified to less angle (up to 4 degrees). Collimating lens with smallest angle (2 degrees) are available, but their diameter is minimum 50 mm, so they aren’t useful for us.

5. Rechargeable batteries for underwater lights

What kind of battery do you need? Several types of batteries are suitable for the underwater LED-based lights. Among them Li-Ion (lithium-ion) and Li-Po (lithium-polymer), Ni-MH (nickel-metal hydride) and NiCd (nickel-cadmium). NiCd have low capacity and distinct “memory effect”, Ni-MH have higher capacity and less “memory effect”. Li-ion and Li-Po batteries have the biggest capacity, moreover they haven’t “memory effect” at all, so you can charge them as you need without causing damages, but they are more expensive than Ni-MH.